We have previously demonstrated the feasibility of using the murine version of the monoclonal antibody BrE-3 for high-dose radioimmunotherapy in patients with breast cancer (CaRes55:5921s, 1995). We have now initiated a Phase I study of a single injection of 90-Y labeled MX-DTPA-humanized-BrE-3 followed by G-CSF mobilized autologous hematopoietic stem cells support in patients with refractory metastatic breast cancer followed by systemic G-CSF. 111-In-labeled hu-BrE is co-injected for imaging and dosimetry purposes. Dose escalation continues until the maximum tolerated dose is reached.